RC transfer function example?

[Megabrein]

Can anyone give me an example on how to setup the transfer function block (in Multisim 9) for a simple low pass filter? [ Transfer function is 1/(1+sRC) ] I could not find any examples on this subject. Any help would be greatly appreciated.

[lacy]

Try right clicking on the component and hit properties. Then in the properties dialog box find the "info" button. Thuis will take you to the help file inside Multisim and it will explain how it is used.. I looked into it some. The one thing I found is that it will not allow formulas to be used in either the denominator or numerator. The values have to be numbers and no sysmbols like +,- or *. So this means that you must solve your equation at least for the denominator in order to get this to work. You may have already known this, but I thought I would let you know what I observed.

I do not know if this is what you need or not, but it is all I know about it.

[lacy]

I tried the example in the help file and I couldn't get it to work for me (it is probably me not doing something right).. Maybe NI can post a better example and show the configuartion of the coefficients better than the help file. I don't use these enough to know much about them.

Message Edited by lacy on 05-27-2008 09:18 PM

[jtilley]

The example in the help file is incorrect.

The formula for a single pole low pass filter is, as you say, 1/[1+sRC]. In Multisim world, this takes on the form of A0/[B0+sB1]. So, what you want to do is set A0=1, B0=1 (Default) and then B1 will be your 'RC'.

Now, most of the time, you want your filter to have a specific corner frequency, the point in the magnitude response that is down 3dB. As an example, I want a single pole low pass filter with a Fc of 10kHz. To get this, I need to set my 'RC' (or B1) to 1/(2*pi*10,000). The 2*pi accounts for converting from Hz to rad/sec. So, you will set B1 equal to 159.1549431e-6. Hook up an input source to your transfer function block and the bode plotter to the output and you will see your result.

I've attached an example Multisim V9 file.

Message Edited by jtilley on 05-30-2008 04:25 PM

[lacy]

Thanks for that information. I knew the formulas, but not how ro enter them into the parameters section and the help file is of no real help as you pointed out.

I think there should be a more in depth technical reference manual for some of these functions and components. This way there would be no misunderstanding on how they function, how to properly set up the parameters, and how to incorporate them into a working ciruit. I think that would make things a little easier for users. I personally have not used these "function blocks" so I really had no idea on how to set the parameters and the fact that the help file was incorrect did make things worse.

Thanks again for your help in this matter. It is greatly appreciated..

[Megabrein]

Jtilley, many thanks for your explanation! You even attached an MS9 file...which i'll give a go right away. Again, thanks!